Problem: Simplify the following expression: $y = \dfrac{-3x^2- 8x+35}{-3x + 7}$
Explanation: First use factoring by grouping to factor the expression in the numerator. This expression is in the form ${A}x^2 + {B}x + {C}$ First, find two values, $a$ and $b$ , so: $ \begin{eqnarray} {ab} &=& {A}{C} \\ {a} + {b} &=& {B} \end{eqnarray} $ In this case: $ \begin{eqnarray} {ab} &=& {(-3)}{(35)} &=& -105 \\ {a} + {b} &=& &=& {-8} \end{eqnarray} $ In order to find ${a}$ and ${b}$ , list out the factors of $-105$ and add them together. Remember, since $-105$ is negative, one of the factors must be negative. The factors that add up to ${-8}$ will be your ${a}$ and ${b}$ When ${a}$ is ${7}$ and ${b}$ is ${-15}$ $ \begin{eqnarray} {ab} &=& ({7})({-15}) &=& -105 \\ {a} + {b} &=& {7} + {-15} &=& -8 \end{eqnarray} $ Next, rewrite the expression as $({A}x^2 + {a}x) + ({b}x + {C})$ $ ({-3}x^2 +{7}x) + ({-15}x +{35}) $ Factor out the common factors: $ x(-3x + 7) + 5(-3x + 7)$ Now factor out $(-3x + 7)$ $ (-3x + 7)(x + 5)$ The original expression can therefore be written: $ \dfrac{(-3x + 7)(x + 5)}{-3x + 7}$ We are dividing by $-3x + 7$ , so $-3x + 7 \neq 0$ Therefore, $x \neq \frac{7}{3}$ This leaves us with $x + 5; x \neq \frac{7}{3}$.